An integrated circuit often has a main power supply (VCC) and a backup power supply. One purpose of a backup supply is to allow the circuit to continue to function when the main power supply is interrupted or drops below a preset voltage level for some reason. A switchover circuit is used to switch the internal power (Vdd) of the circuit between the main supply and the backup supply. It is desirable to have a switchover circuit that uses internal power (Vdd) as its supply voltage and that uses a low amount of power. As circuits continue to decrease in size, it is also desirable for a switchover circuit that occupies a small area.
One example of a backup power supply for an integrated circuit is a battery. In this instance, a switchover circuit that uses a low amount of power would be useful to extend battery life. For example, a switchover circuit of the type discussed above would be useful in electronic devices in which the power from the net is used together with a backup supply (e.g. clock radio, e-metering, etc).
In one example of a switchover circuit, the switchover circuit includes two comparators that compare the main power supply voltage (VCC) to the backup battery voltage (VBAT) and to a battery threshold value (VBMT). The comparators are powered with internal power (Vdd), and the battery threshold value (VBMT) is generated by a reference circuit that is also powered with internal power (Vdd). The switchover circuit switches to the backup battery when the voltage of the main supply (VCC) is less than both the battery voltage (VBAT) and the battery threshold value (VBMT), and switches back to the main power supply when the main supply voltage is greater than the battery voltage or the battery threshold value. However, this type of switchover circuit has several limitations. For example, the battery threshold value (VBMT) has to be specified at a value lower than the minimum voltage of the internal power (Vdd) because the battery threshold value is generated by a reference circuit that is powered with internal power.
One possible solution to these problems is to power the comparators and the reference circuit that generates the battery threshold value (VBMT) with a voltage level higher than the internal power (Vdd). The higher voltage level can be generated by using a doubler to increase the voltage level of the internal power (Vdd). However, the addition of a doubler can increase the size and the power consumption of the switchover circuit.
In another example of a switchover circuit, the comparators are supplied by internal power (Vdd), but they compare partitioned levels of the inputs (e.g., VCC/2, VBAT/2 and VBMT/2). In this case, a partitioned level of the battery threshold value (VBMT) is generated; therefore, the battery threshold value can be greater than the minimum voltage of the internal power (Vdd). Previous approaches have used a bandgap source or a resister divider to generate the partitioned voltage levels for the comparators. However, a bandgap source and a resister divider can result in an increase in the power consumption and in the size of the switchover circuit.
These and other issues have presented challenges to the implementation of switchover circuits.